In literature there have been described processes for modifying with derivatives of the silanic type the surface of materials, such as SiO.sub.2, TiO.sub.2, aluminas, etc., utilized as fillers and as non-colored pigments. Thus, in the case of the fillers employed in many thermoplastic and thermosetting materials, in order to improve the adhesion between the filler and the other components. The filler or pigment so obtained may be finally coloured with conventional dyes. There are substantially described usual reactive or acid, polysulphonic, and in particular not quinophthalonic dyes, not containing silane residues, soluble in water, these dyes requiring however methods of preparing the relevant fillers or colored pigments different from the methods described in the present invention.